1. The Field of the Invention
This invention relates to apparatus for purifying a liquid by reverse osmosis, and more particularly to such apparatus employing a semipermeable membrane cartridge reciprocated within a pressure resistant vessel.
2. The Prior Art
In my U.S. Pat. No. 4,070,280 issued Jan. 24, 1978, I have described and claimed reverse osmosis apparatus in which a hollow tube passes slidably and sealingly through one end of a pressure resistant vessel or container and is connected to the permeate collector tube of a semipermeable membrane cartridge slidably enclosed within the pressure resistant vessel. Permeate may be withdrawn through the hollow tube which also serves as a mechanism for importing reciprocal motion to the semipermeable membrane cartridge. Pressurized feed water is introduced into the pressure resistant vessel near one end, and concentrate or brine is released near the other end, the release pressure being controlled by a restrictor or other device to maintain reverse osmosis operating pressure within the pressure resistant vessel. This arrangement produces an improvement in operation because boundary layer conditions at the membrane surface are mitigated or alleviated by the turbulence and flow of the feed water produced by reciprocation of the semipermeable membrane cartridge. However, the force necessary to move such a semipermeable membrane cartridge in one direction can be different from that required to move it in the opposite direction. This occurs because the effective pressurized area of the end of the semipermeable membrane cartridge to which the actuating hollow tube is attached is less than that of the other end of the cartridge, due to the area covered by the cross section of the hollow tube. Even though the hollow tube exterior diameter may be small, the resulting force difference may be substantial if the liquid pressure inside the pressure resistant container is high, and this may be of the order of up to 1,000 psi. Apparatus that would provide a balanced load imparted by contained liquid at each end of the semipermeable membrane cartridge would provide a valuable improvement.